The invention relates to a transformer having a primary coil and a secondary coil, with a ferromagnetic core inserted in a coil former.
Transformers of this type are known in the art and serve the purpose of transforming alternating current. The field of application of such transformers covers AC/DC or DC/DC converters and power supply units in particular. In such applications, it is of considerable importance to isolate the input circuit and output circuit galvanically from each other. The isolation of the galvanic regions in most cases consists of plastics material. This plastics material is capable of withstanding certain thermal loads. In the event of a fault, however, the temperature of the transformer may rise to a value at which the coil former or the winding insulation melts. The short-circuit then occurring between the primary winding and the secondary winding destroys the galvanic isolation.
To avoid such destruction of the galvanic isolation, it has been proposed in the prior art to monitor the temperature of the transformer housings by thermocouples. A common precautionary measure is also to introduce a fuse into the primary circuit.
The invention is based on the object of developing a transformer of the generic type to improve the electrical isolation protection.
According to the invention the alternating current resistance of the primary coil decreases on reaching a triggering temperature which lies above the operating temperature of the transformer but below the softening temperature of the coil former and/or insulation of the coil winding. For this purpose, the coil core preferably consists at least partially of a material with a magnetic permeability which drops when the triggering temperature is exceeded. For this purpose, the core preferably consists of a ferrite. This ferrite core is to have at least one region in which the Curie temperature is lower than the softening temperature of the coil core and/or coil insulation. If the temperature of the coil core rises below the Curie temperature, the permeability of the coil core remains virtually constant or increases slightly. On reaching the Curie temperature, the relative permeability of the material drops abruptly to the value 1. This abrupt drop takes place over only a few degrees. The coil core preferably comprises two core parts. In this case it is adequate if one of the two core parts consists of a material in which the relative permeability changes abruptly at the temperature mentioned above. It is adequate if only a subregion of the core has these properties. The Curie temperature of the region or of the entire core preferably lies between 120 and 220xc2x0 C. The invention also relates to a power supply unit or a voltage converter with a transformer in which the alternating current resistance of the primary coil decreases on reaching a triggering temperature which lies above the operating temperature, this triggering temperature lying below the softening temperature of the coil former and/or insulation of the coil winding. In this case, a fuse is connected in the primary circuit, the triggering current of the said fuse being lower than the current flowing through the primary coil when the triggering temperature is exceeded. In a preferred embodiment, the voltage converter is a DC/DC converter, in which the alternating current is generated by a switching IC.